The conventional wheelchair is typically comprised of two large diameter, ground engaging, narrow width rear wheels mounted onto an axle positioned beneath the seat portion of the chair, with a pair of smaller diameter, narrow width ground engaging wheels positioned forwardly of the rear wheels. The front wheels are pivotally mounted onto vertical stanchions and give the user of the chair the ability to steer, at least to some degree, the chair. Typically, the occupant of the wheelchair is seated in a conventional fashion between the rear wheels, with the lower portion of the operator's legs being more or less perpendicular to the ground and supported in a footrest attached thereto.
The operator of a conventional wheelchair as described above, when attempting to using it off road, is faced with a number of practical problems. For example, almost all commercially available chairs utilize relatively narrow, solid or pneumatic tires on both the front and rear wheels. When utilized in sandy, loose or unstable terrain, such wheels tend to sink into the underlying terrain and loose traction resulting in the occupant being left stranded. Further, such wheels are subject to slicing or puncturing by rocks, branches and the like. To lessen the puncture and traction loss problems several solutions have been propounded.
One such solution has been to mount an additional rear wheel to each side of the existing rear wheels, thus widening the effective tread and bearing ability of those wheels. Another has been to utilize a flattened metal rear wheel to accomplish the same purpose. While utilization of two adjacent wheels provides better traction, the puncture problem continues to exist and there are still some problems with sinking into unstable terrain. The metal wheel, while less likely to sink, typically lacks traction and, particularly when the user must provide the motive power for the chair, has proven to be quite uncomfortable for the user to grasp.
A further problem encountered in such situations is that steering of the vehicle with the front wheels becomes awkward and difficult as the wheels become engaged with the unstable ground fully losing the ability to steer and support the wheelchair. As a result the wheelchair user is generally limited to using it on hard, unyielding surfaces, both indoors and out.